Driving methods for color display devices

ABSTRACT

The present invention provides driving methods for electrophoretic color display devices. The backplane system used for the driving methods is found to be simpler which renders color display devices more cost effective. More specifically, the driving method comprises first driving all pixels towards a color state by modulating only the common electrode, followed by driving all pixels towards their desired color states by maintaining the common electrode grounded and applying different voltages to the pixel electrodes.

REFERENCE TO RELATED APPLICATIONS

This application is a division of copending application Ser. No.14/279,125, filed May 15, 2014 (Publication No. 2014/0340734), whichclaims the priority of U.S. Provisional Application No. 61/824,928,filed May 17, 2013, the contents of which is incorporated herein byreference in its entirety.

BACKGROUND OF INVENTION

The present invention is directed to driving methods for color displaydevices. The methods can greatly reduce complexity of the active matrixbackplane used for this type of display devices.

In order to achieve a color display, color filters are often used. Themost common approach is to add color filters on top of black/whitesub-pixels of a pixellated display to display the red, green and bluecolors. The biggest disadvantage of such a technique is that the whitelevel is normally substantially less than half of that of a black andwhite display, rendering it an unacceptable choice for display devices,such as e-readers or displays that need well readable black-whitebrightness and contrast.

SUMMARY OF INVENTION

A first aspect of the invention is directed to a driving method for adisplay device comprising

-   -   (i) an electrophoretic fluid which fluid comprises a first type        of particles, a second type of particles and a third type of        particles, all of which are dispersed in a solvent or solvent        mixture, wherein the first type of particles carry a charge        polarity while the second and third types of particles carry        opposite charge polarity, and    -   (ii) a plurality of pixels wherein each pixel is sandwiched        between a common electrode and a pixel electrode, which method        comprises        -   a) applying no voltage to the pixel electrodes and applying            a high voltage to the common electrode wherein the high            voltage has a polarity opposite of the charge polarity of            the first type of particles, to drive all pixels towards the            color state of the first type of particles;        -   b) applying no voltage to the pixel electrodes and applying            a low voltage to the common electrode wherein the low            voltage has a polarity opposite of the charge polarity of            the third type of particles, to drive all pixels towards the            color state of the third type of particles; and        -   c) maintaining the common electrode grounded and applying            different voltages to the pixel electrodes to drive pixels            towards their desired color states.

In one embodiment, in step (c), no voltage is applied to the pixelelectrodes to maintain the pixels in the color state of the third typeof particles.

In one embodiment, in step (c), a high voltage is applied to the pixelelectrodes wherein the high voltage has the same polarity as the secondtype of particles to drive the pixels towards the color state of thesecond type of particles.

In one embodiment, in step (c), a high voltage is applied to the pixelelectrodes wherein the high voltage has the same polarity as the firsttype of particles to drive the pixels towards the color state of thefirst type of particles.

In one embodiment, the method further comprises a shaking waveform priorto step (a).

In one embodiment, the first type of particles is negatively charged andthe second and third types of particles are positively charged.

In one embodiment, the first type of particles is white particles, thesecond type of particles is black particles and the third type ofparticles is non-white and non-black particles.

A second aspect of the invention is directed to a driving method for adisplay device comprising

-   -   (i) an electrophoretic fluid which fluid comprises a first type        of particles, a second type of particles, a third type of        particles and a fourth type of particles, all of which are        dispersed in a solvent or solvent mixture, wherein the first and        second types of particles are oppositely charged and the third        and fourth types of particles are oppositely charged, and    -   (ii) a plurality of pixels wherein each pixel is sandwiched        between a common electrode and a pixel electrode, which method        comprises        -   a) applying no voltage to the pixel electrodes and applying            a high voltage to the common electrode wherein the high            voltage has a polarity opposite of the charge polarity of            the second type of particles, to drive all pixels towards            the color state of the second type of particles;        -   b) applying no voltage to the pixel electrodes and applying            a low voltage to the common electrode wherein the low            voltage has a polarity opposite of the charge polarity of            the third type of particles, to drive all pixels towards the            color state of the third type of particles; and        -   c) maintaining the common electrode grounded and applying            different voltages to the pixel electrodes to drive pixels            towards their desired color states.

In one embodiment, in step (c), no voltage is applied to the pixelelectrodes to maintain the pixels in the color state of the third typeof particles.

In one embodiment, in step (c), a high voltage is applied to the pixelelectrodes wherein the high voltage has the same polarity as the firsttype of particles to drive the pixels towards the color state of thefirst type of particles.

In one embodiment, in step (c), a high voltage is applied to the pixelelectrodes wherein the high voltage has the same polarity as the secondtype of particles to drive the pixels towards the color state of thesecond type of particles.

In one embodiment, in step (c), a high voltage is applied to the pixelelectrodes wherein the high voltage has the same polarity as the firsttype of particles, followed by applying a low voltage to the pixelelectrodes wherein the low voltage has the same polarity as the fourthtype of particles to drive the pixels towards the color state of thefourth type of particles.

In one embodiment, the method further comprises a shaking waveform priorto step (a).

In one embodiment, the first and third types of particles are positivelycharged and the second and fourth types of particles are negativelycharged.

In one embodiment, the first type of particles is black particles, thesecond type of particles is yellow particles, the third type ofparticles is red particles and the fourth type of particles is whiteparticles.

In one embodiment, the first type of particles is high positiveparticles, the second type of particles is high negative particles, thethird type of particles is low positive particles and the fourth type ofparticles is low negative particles.

A third aspect of the invention is directed to a driving method for acolor display device comprising a plurality of pixels, wherein each ofthe pixels is sandwiched between a common electrode and a pixelelectrode, the method comprises:

-   -   a) driving all pixels towards a color state by modulating only        the common electrode; and    -   b) driving all pixels towards their desired color states by        maintaining the common electrode grounded and applying different        voltages to the pixel electrodes.

In one embodiment, the method further comprises a shaking waveform.

A fourth aspect of the invention is directed to a backplane system fordriving a display device comprising an electrophoretic fluid wherein thefluid comprises a first type of particles, a second type of particlesand a third type of particles, all of which are dispersed in a solventor solvent mixture, wherein the first type of particles carry a chargepolarity while the second and third types of particles carry oppositecharge polarity, which backplane system has only three levels ofvoltage, 0V, a high positive voltage and a high negative voltage.

A fifth aspect of the invention is directed to a backplane system fordriving a display device comprising an electrophoretic fluid wherein thefluid comprises a first type of particles, a second type of particles, athird type of particles and a fourth type of particles, all of which aredispersed in a solvent or solvent mixture, wherein the first and secondtypes of particles are oppositely charged and the third and fourth typesof particles are oppositely charged, which backplane system has onlyfour levels of voltage, 0V, a high positive voltage, a high negativevoltage and a low positive voltage or a low negative voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a display layer of the present invention.

FIG. 2 depicts an electrophoretic fluid comprising three types ofparticles.

FIGS. 3A-3B illustrate the driving sequence of the three particle fluidsystem.

FIGS. 4A, 4B and 5 illustrate a driving method of the present inventionfor the three particle fluid system.

FIG. 6 depicts an electrophoretic fluid comprising four types ofparticles.

FIGS. 7A-7C illustrate the driving sequence of the four particle fluidsystem.

FIGS. 8A, 8B and 9 illustrate a driving method of the present inventionfor the four particle fluid system.

FIGS. 10A, 11A and 12A are diagrams for implementation of the presentdriving methods.

FIGS. 10B, 11B and 12B are diagrams for prior art driving methods.

DETAILED DESCRIPTION General

A display fluid of the present invention may comprise three or fourtypes of particles. The multiple types of particles may be of any colorsas long as the colors are visually distinguishable. In the fluid, theparticles are dispersed in a solvent or solvent mixture.

For white particles, they may be formed from an inorganic pigment, suchas TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ or the like.

For black particles, they may be formed from CI pigment black 26 or 28or the like (e.g., manganese ferrite black spinel or copper chromiteblack spinel) or carbon black.

The colored particles (non-white and non-black) may be of a color suchas red, green, blue, magenta, cyan or yellow. The pigments for this typeof particles may include, but are not limited to, CI pigment PR 254,PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY138, PY150, PY155 andPY20. These are commonly used organic pigments described in color indexhandbooks, “New Pigment Application Technology” (CMC Publishing Co, Ltd,1986) and “Printing Ink Technology” (CMC Publishing Co, Ltd, 1984).Specific examples include Clariant Hostaperm Red D3G 70-EDS, HostapermPink E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm BlueB2G-EDS, Hostaperm Yellow H4G-EDS, Hostaperm Green GNX, BASF Irgazinered L 3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD; SunChemical phthalocyanine blue, phthalocyanine green, diarylide yellow ordiarylide AAOT yellow.

In addition to the colors, the multiple types of particles may haveother distinct optical characteristics, such as optical transmission,reflectance, or luminescence or, in the case of displays intended formachine reading, pseudo-color in the sense of a change in reflectance ofelectromagnetic wavelengths outside the visible range.

A display layer utilizing a display fluid of the present invention, asshown in FIG. 1, has two surfaces, a first surface (13) on the viewingside and a second surface (14) on the opposite side of the first surface(13). The display fluid is sandwiched between the two surfaces. On theside of the first surface (13), there is a common electrode (11) whichis a transparent electrode layer (e.g., ITO), spreading over the entiretop of the display layer. On the side of the second surface (14), thereis an electrode layer (12) which comprises a plurality of pixelelectrodes (12 a).

The pixel electrodes are described in U.S. Pat. No. 7,046,228, thecontent of which is incorporated herein by reference in its entirety. Itis noted that while active matrix driving with a thin film transistor(TFT) backplane is mentioned for the layer of pixel electrodes, thescope of the present invention encompasses other types of electrodeaddressing as long as the electrodes serve the desired functions.

Each space between two dotted vertical lines in FIG. 1 denotes a pixel.As shown, each pixel has a corresponding pixel electrode. An electricfield is created for a pixel by the potential difference between avoltage applied to the common electrode and a voltage applied to thecorresponding pixel electrode.

The multiple types of particles may have different charge levels. In oneembodiment, the weaker charged particles have charge intensity beingless than about 50%, or about 5% to about 30%, the charge intensity ofthe stronger charged particles. In another embodiment, the weakercharged particles have charge intensity being less than about 75%, orabout 15% to about 55%, the charge intensity of the stronger chargedparticles. In a further embodiment, the comparison of the charge levelsas indicated applies to two types of particles having the same chargepolarity.

The charge intensity may be measured in terms of zeta potential. In oneembodiment, the zeta potential is determined by Colloidal DynamicsAcoustoSizer IIM with a CSPU-100 signal processing unit, ESA EN# Attnflow through cell (K:127). The instrument constants, such as density ofthe solvent used in the sample, dielectric constant of the solvent,speed of sound in the solvent, viscosity of the solvent, all of which atthe testing temperature (25.degree. C.) are entered before testing.Pigment samples are dispersed in the solvent (which is usually ahydrocarbon fluid having less than 12 carbon atoms), and diluted tobetween 5-10% by weight. The sample also contains a charge control agent(Solsperse®. 17000, available from Lubrizol Corporation, a BerkshireHathaway company), with a weight ratio of 1:10 of the charge controlagent to the particles. The mass of the diluted sample is determined andthe sample is then loaded into the flow through cell for determinationof the zeta potential.

If there are two pairs of high-low charge particles in the same fluid,the two pairs may have different levels of charge differentials. Forexample, in one pair, the low positively charged particles may have acharge intensity which is 30% of the charge intensity of the highpositively charged particles. In another pair, the low negativelycharged particles may have a charge intensity which is 50% of the chargeintensity of the high negatively charged particles.

The solvent in which the multiple types of particles are dispersed isclear and colorless. It preferably has a low viscosity and a dielectricconstant in the range of about 2 to about 30, preferably about 2 toabout 15 for high particle mobility. Examples of suitable dielectricsolvent include hydrocarbons such as isopar, decahydronaphthalene(DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, siliconfluids, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenatedsolvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotri fluoride,chloropentafluorobenzene, dichlorononane or pentachlorobenzene, andperfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company,St. Paul Minn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoroethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galdenfrom Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont,Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

In the present invention, at least one type of particles may demonstratean electric field threshold. In one embodiment, one type of the highercharged particles has an electric field threshold.

The term “electric field threshold”, in the context of the presentinvention, is defined as the maximum electric field that may be appliedfor a period of time (typically not longer than 30 seconds, preferablynot longer than 15 seconds), to a group of particles, without causingthe particles to appear at the viewing side of a pixel, when the pixelis driven from a color state different from the color state of the groupof particles. The term “viewing side”, in the present application,refers to the first surface in a display layer where images are seen bythe viewers.

The electric field threshold is either an inherent characteristic of thecharged particles or an additive-induced property.

In the former case, the electric field threshold is generated, relyingon certain attraction force between oppositely charged particles orbetween particles and certain substrate surfaces.

In the case of additive-induced electric field threshold, a thresholdagent which induces or enhances the threshold characteristics of anelectrophoretic fluid may be added. The threshold agent may be anymaterial which is soluble or dispersible in the solvent or solventmixture of the electrophoretic fluid and carries or induces a chargeopposite to that of the charged particles. The threshold agent may besensitive or insensitive to the change of applied voltage. The term“threshold agent” may broadly include dyes or pigments, electrolytes orpolyelectrolytes, polymers, oligomers, surfactants, charge controllingagents and the like.

Three Particle System

FIG. 2 depicts a three particle fluid system as described in US2014/0092466; the content of which is incorporated herein by referencein its entirety.

The electrophoretic fluid comprises three types of particles dispersedin a dielectric solvent or solvent mixture. For ease of illustration,the three types of particles may be referred to as a first type ofparticles, a second type of particles and a third type of particles. Asan example shown in FIG. 2, the first type of particles is whiteparticles (W); the second type of particles is black particles (K); andthe third type of particles is red particles (R). The third type ofparticles can be any colors of non-white and non-black.

Two of the three types of particles (i.e., the first and second types ofparticles) have opposite charge polarities and the third type ofparticles carries the same charge polarity as one of the other two typesof particles. For example, if the black particles are positively chargedand the white particles are negatively charged, and then the redparticles are either positively charged or negatively charged.

FIG. 3 demonstrates the driving sequence of this type of color displaydevice. For illustration purpose, the white particles (W) are negativelycharged while the black particles (K) are positively charged. The redparticles (R) carry the same charge polarity as the black particles (K).

Because of the attraction force between the black and white particles,the black particles (K) are assumed to have an electric field thresholdof λV. Therefore, the black particles would not move to the viewing sideif an applied voltage potential difference is λV or lower.

The red particles carry a charge weaker than that of the black and whiteparticles. As a result, the black particles move faster than the redparticles (R), when an applied voltage potential is higher than λVbecause of the stronger charge carried by the black particles.

In FIG. 3A, a high positive voltage potential difference, +hV, isapplied. In this case, the white particles (W) move to be near or at thepixel electrode (32 a) and the black particles (K) and the red particles(R) move to be near or at the common electrode (31). As a result, ablack color is seen at the viewing side. The red particles (R) movetowards the common electrode (31); however because they carry lowercharge, they move slower than the black particles (K).

In FIG. 3B, when a high negative potential difference, −hV, is applied,the white particles (W) move to be near or at the common electrode (31)and the black particles (K) and the red particles (R) move to be near orat the pixel electrode (32 a). As a result, a white color is seen at theviewing side. The red particles (R) move towards the pixel electrodebecause they are also positively charged. However, because of theirlower charge intensity, they move slower than the black particles.

In FIG. 3C, a low positive voltage potential difference, +λV, is appliedto the pixel of FIG. 3A (i.e., driving from the white color state). Inthis case, the negatively charged white particles (W) in FIG. 3A movetowards the pixel electrode (32 a). The black particles (K) move littlebecause of their electric field threshold being λV. Due to the fact thatthe red particles (R) do not have a significant electric fieldthreshold, they move to be near or at the common electrode (31) and as aresult, a red color is seen at the viewing side.

It is noted that the lower voltage (+λV or −λV) applied usually has amagnitude of about 5% to about 50% of the magnitude of the full drivingvoltage required to drive the pixel from the black state to the whitestate (−hV) or from the white state to the black state (+hV). In oneembodiment, +hV and −hV may be +15V and −15V, respectively and +λV and−λV may be +3V and −3V, respectively. In addition, it is noted that themagnitudes of +hV and −hV may be the same or different. Likewise, themagnitude of +λV and −λV may be the same or different.

The term “driving voltage potential difference” refers to the voltagedifference between the common electrode and a pixel electrode. In theprevious driving method, the common electrode shared by all pixelsremains grounded and each pixel is driven by the voltage applied to thecorresponding pixel electrode. If such an approach is used to drive thefluid system as described in FIGS. 2 and 3, the backplane system wouldneed to have each pixel electrode at least four different levels ofvoltages, 0V, +hV, −hV and +λV. Such a backplane system is costly toimplement, which is explained in a section below.

The present inventors now propose a new driving method where thebackplane system is simplified while color states of high quality canstill be displayed.

FIGS. 4A and 4B illustrate the initial step of the present drivingmethod and this step is applied to all pixels. A shaking waveform isfirst applied, after which in phase I, the common electrode shared byall pixels is applied a +hV while all pixel electrodes are at 0V,resulting in a driving voltage potential difference of −hV for allpixels, which drive all of them towards the white state (see FIG. 3A).In phase II, −λV is applied to the common electrode while all pixelelectrodes still remain at 0V, resulting in a driving voltage potentialdifference of +λV, which drives all pixels towards the red state (seeFIG. 3c ). In this initial step, all pixel electrodes remain at 0V whilethe common electrode is modulated, switching from +hV to −λV.

The shaking waveform applied before phase I consists of a pair ofopposite driving pulses for many cycles. For example, the shakingwaveform may consist of a +15V pulse for 20 msec and a −15V pulse for 20msec and such a pair of pulses is repeated for 50 times. The total timeof such a shaking waveform would be 2000 msec.

With this added shaking waveform, the color state (i.e., red) can besignificantly better than that without the shaking waveform, on bothcolor brightness and color purity. This is an indication of betterseparation of the white particles from the red particles as well as theblack particles from the red particles.

Each of the driving pulses in the shaking waveform is applied for notexceeding half of the driving time from the full black state to the fullwhite state or vice versa. For example, if it takes 300 msec to drive adisplay device from a full black state to a full white state or viceversa, the shaking waveform may consist of positive and negative pulses,each applied for not more than 150 msec. In practice, it is preferredthat the pulses are shorter.

FIG. 5 illustrates the next step of the driving method. In this step,all pixels are driven simultaneously to their desired color states. Inthis step, the common electrode is grounded at 0V while differentvoltages are applied to the pixel electrodes. For pixels that remain inthe red state, no voltage is applied to the corresponding pixelelectrodes, resulting in no driving voltage potential difference. Forpixels to be in the black state, a +hV is applied to the correspondingpixel electrodes. For pixels to be in the white state, a −hV is appliedto the corresponding pixel electrodes.

With this driving method in which the common electrode is modulated inthe initial step, the backplane system would only need to have eachpixel electrode at three different levels of voltage, 0V, +hV and −hV,which is much simplified than the backplane system used in the previousmethod.

FIG. 10A is a simplified diagram illustrating the implementation of thepresent driving method. As shown, there are three different levels ofvoltage, 0V, +hV and −λV, which may be applied to the common electrode(Vcom) and there are three different levels of voltage, 0V, +hV and −hV,which may be applied to a pixel electrode.

FIG. 10B is a diagram illustrating the corresponding prior art method.In this diagram, there are three levels of voltage, +hV, −hV and +λV,which may be applied to a pixel electrode. Commercially available TFTbackplane usually only has source IC which supports 0V, +hV and −hV.Therefore if the prior art method is utilized, there would be the needto modify the source IC to support an additional voltage option of +λVapplied to the pixel electrode.

Four Types of Particles

FIG. 6 depicts an alternative display device in which theelectrophoretic fluid comprises four types of particles dispersed in adielectric solvent or solvent mixture, as described in U.S. ProvisionalApplication No. 61/824,887, which is incorporated herein by reference inits entirety. For ease of illustration, the four types of particles maybe referred to as a first type of particles, a second type of particles,a third type of particles and a fourth type of particles. As an exampleshown in FIG. 6, the first type of particles is black particles (K); thesecond type of particles is yellow particles (Y); the third type ofparticles is red particles (R); and the fourth type of particles iswhite particles (W).

In this example, the black and yellow particles carry opposite chargepolarities. For example, if the black particles are positively charged,the yellow particles are negatively charged. The red and white particlesare also oppositely charged. However the charges carried by the blackand yellow particles are stronger than the charges carried by the redand white particles.

For example, the black particles (K) carry a high positive charge; theyellow particles (Y) carry a high negative charge; the red (R) particlescarry a low positive charge; and the white particles (W) carry a lownegative charge. The driving sequence of this type of color displaydevice is shown in FIG. 7.

In FIG. 7A, when a high negative voltage potential difference (e.g.,−hV) is applied to a pixel, the yellow particles (Y) are pushed to thecommon electrode (71) side and the black particles (K) are pulled to thepixel electrode (72 a) side. The red (R) and white (W) particles, due totheir lower charge levels, move slower than the higher charged black andyellow particles and therefore they stay between the common electrodeand the pixel electrode, with white particles above the red particles.As a result, a yellow color is seen at the viewing side.

Also in FIG. 7A, when a high positive voltage potential difference(e.g., +hV) is applied to the pixel, the particle distribution would bereversed and as a result, a black color is seen at the viewing side.

In FIG. 7B, when a lower positive voltage potential difference (e.g.,+λV) is applied to the pixel in its yellow state, the yellow particles(Y) move towards the pixel electrode (72 a) while the black particles(K) move towards the common electrode (71). However, when they meetwhile moving, because of their strong attraction to each other, theystop moving and remain between the common electrode and the pixelelectrode. The lower charged (positive) red particles (R) move all theway towards the common electrode (71) side (i.e., the viewing side) andthe lower charged (negative) white particles (W) move towards the pixelelectrode (72 a) side. As a result, a red color is seen.

In FIG. 7C, when a lower negative voltage potential difference (e.g.,−λV) is applied to the pixel in its black state, the black particles (K)move towards the pixel electrode (72 a) while the yellow particles (Y)move towards the common electrode (71). When the black and yellowparticles meet, because of their strong attraction to each other, theystop moving and remain between the common electrode and the pixelelectrode. The lower charged (negative) white particles (W) move all theway towards the common electrode side (i.e., the viewing side) and thelower charged (positive) red particles (R) move towards the pixelelectrode side. As a result, a white color is seen.

It is also noted that in FIGS. 7B and 7C, while the low driving voltagesapplied (+λV or −λV) are not sufficient to separate the stronger chargedblack and yellow particles, they, however, are sufficient to separate,not only the two types of oppositely charged particles of lower chargeintensity, but also the lower charged particles from the strongercharged particles of opposite charge polarity.

It is noted that the lower voltage (+λV or −λV) applied usually has amagnitude of about 5% to about 50% of the magnitude of the full drivingvoltage required to drive the pixel from the black state to the yellowstate (−hV) or from the yellow state to the black state (+hV). In oneembodiment, +hV and −hV may be +15V and −15V, respectively and +λV and−λV may be +3V and −3V, respectively. In addition, it is noted that themagnitudes of +hV and −hV may be the same or different. Likewise, themagnitude of +λV and −λV may be the same or different.

FIGS. 8A and 8B illustrate the initial step of the present drivingmethod for the four particle system and this step is applied to allpixels. In phase I, the common electrode shared by all pixels is applieda +hV while all pixel electrodes are at 0V, resulting in a drivingvoltage potential difference of −hV for all pixels which drive all ofthem to the yellow state. In phase II, −λV is applied to the commonelectrode while all pixel electrodes still remain at 0V, resulting in adriving voltage potential difference of +λV, which drives all pixels tothe red state. In this initial step, all pixel electrodes remain at 0Vwhile the common electrode is modulated, switching from +hV to −λV.

The shaking waveforms as described for FIG. 4 may also be applied inthis case.

FIG. 9 illustrates the next step of the driving method. In this step,the common electrode is grounded at 0V while different voltages areapplied to the pixel electrodes. For pixels to remain in the red state,no voltage is applied to the corresponding pixel electrodes, resultingin no driving voltage potential difference. For pixels to be in theblack state, a +hV is applied to the corresponding pixel electrodes. Forpixels to be in the yellow state, a −hV is applied to the correspondingpixel electrodes. For pixels to be in the white state, a +hV, followedby a −λV is applied to the corresponding pixels.

With this driving method in which the common electrode is modulated inthe initial step, the backplane system would only need to have eachpixel electrode at four different levels of voltage, 0V, +hV, −hV and−λV which is much simplified than the backplane system used in theprevious method in which the system would be required to have each pixelat five different levels of voltage, 0V, +hV, −hV, +λV and −λV.

FIG. 11A is a simplified diagram illustrating the implementation of thepresent driving method. As shown, there are three different levels ofvoltage, 0V, +hV and −λV, which may be applied to the common electrode(Vcom) and there are four different levels of voltage, 0V, +hV, −hV and−λV, which may be applied to the pixel electrode.

FIG. 11B is a diagram illustrating the corresponding prior art method.In this diagram, there are four levels of voltage, +hV, −hV, +λV and−λV, which may be applied to a pixel electrode. Commercially availableTFT backplane usually only has source IC which supports 0V, +hV and −hV.Therefore if the prior art method is utilized, there would be the needto modify the source IC to support one additional voltage option of +λVapplied to the pixel electrode, compared to the present driving method.

In the illustration above, in the initial step, all pixels are driven tothe red state. However it is also possible to drive all pixels to thewhite state in the initial step (by keeping the pixel electrodes groundsand applying a −hV followed by +λV to the common electrode), followed bydriving pixels to be black from white to black (+hV), driving pixels tobe yellow from white to yellow (−hV), and applying no driving voltagepotential difference to white pixels to remain white. The pixels to bered are driven from white to yellow (−hV) and then from yellow to red(+λV). In this scenario, the backplane system would only need to haveeach pixel electrode at four different levels of voltage, 0V, +hV, −hVand +λV.

FIG. 12A is a simplified diagram illustrating the implementation of thepresent driving method. As shown, there are three different levels ofvoltage, 0V, −hV and +λV, which may be applied to the common electrode(Vcom) and there are four different levels of voltage, 0V, +hV, −hV and+λV, which may be applied to the pixel electrode.

FIG. 12B is a diagram illustrating the corresponding prior art method.In this diagram, there are four levels of voltage, +hV, −hV, +λV and−λV, which may be applied to a pixel electrode. Commercially availableTFT backplane usually only has source IC which supports 0V, +hV and −hV.Therefore if the prior art method is utilized, there would be the needto modify the source IC to support one additional voltage option of −λVapplied to the pixel electrode, compared to the present driving method.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, materials, compositions, processes, process stepor steps, to the objective and scope of the present invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

The invention claimed is:
 1. A method of driving a display layer havinga plurality of pixels and a first, viewing surface and a second surfaceon the opposed side of the display layer from the first surface, thedisplay layer being provided with means for applying an electric fieldbetween the first and second surfaces, the display layer furthercomprising an electrophoretic fluid comprising a fluid and first,second, third and fourth types of particles dispersed in the fluid, thefirst, second, third and fourth types of particles having respectivelyfirst, second, third and fourth optical characteristics differing fromone another, the first and third types of particles bear charges of onepolarity and the second and fourth types of particles bear charges ofthe opposite polarity, the method comprising: a) applying to all pixelsa first electric field having a high magnitude, thereby driving thefirst type of particles towards the viewing surface and causing thedisplay layer to display the first optical characteristic at the viewingsurface; b) thereafter applying to all pixels a second electric fieldhaving a lower magnitude than, and a polarity opposite to, the firstelectric field, thereby driving the fourth type of particles towards theviewing surface and causing the display layer to display the fourthoptical characteristic at the viewing surface; and c) thereafterapplying to at least one pixel of the display an electric field to causethe pixel to display an optical characteristic different from theoptical characteristic displayed at the end of step b).
 2. The method ofclaim 1 wherein step c) is effected by at least one of the following:(i) applying to at least one pixel of the display a high electric fieldhaving the same polarity as the first electric field, thereby causingthe pixel to display the first optical characteristic; (ii) applying toat least one pixel of the display a high electric field having theopposite polarity from the first electric field, thereby causing thepixel to display the second optical characteristic; and (iii) applyingto at least one pixel of the display a high electric field having fromthe opposite polarity from the first electric field, followed byapplying to the pixel a low electric field of the same polarity as thefirst electric field, thereby causing the pixel to display the thirdoptical characteristic.
 3. The method of claim 1, further comprisingapplying a shaking waveform prior to step (a).
 4. A method of driving adisplay layer having a plurality of pixels and a first, viewing surfaceand a second surface on the opposed side of the display layer from thefirst surface, the display layer being provided with means for applyingan electric field between the first and second surfaces, the displaylayer further comprising an electrophoretic fluid comprising a fluid andfirst, second and third types of particles dispersed in the fluid, thefirst, second and third types of particles having respectively first,second and third optical characteristics differing from one another, thefirst type of particles bear charges of one polarity and the second andthird types of particles bear charges of the opposite polarity, themethod comprising: a) applying to all pixels a first electric fieldhaving a high magnitude, thereby driving the first type of particlestowards the viewing surface and causing the display layer to display thefirst optical characteristic at the viewing surface; b) thereafterapplying to all pixels a second electric field having a lower magnitudethan, and a polarity opposite to, the first electric field, therebydriving the third type of particles towards the viewing surface andcausing the display layer to display the third optical characteristic atthe viewing surface; and c) thereafter applying to at least one pixel ofthe display an electric field to cause the pixel to display an opticalcharacteristic different from the optical characteristic displayed atthe end of step b).
 5. The method of claim 4 wherein step c) is effectedby at least one of the following: (i) applying to at least one pixel ofthe display a high electric field having the same polarity as the firstelectric field, thereby causing the pixel to display the first opticalcharacteristic; and (ii) applying to at least one pixel of the display ahigh electric field having the opposite polarity from the first electricfield, thereby causing the pixel to display the second opticalcharacteristic.
 6. The method of claim 4, further comprising applying ashaking waveform prior to step (a).
 7. A method of driving a displaylayer having a plurality of pixels and a first, viewing surface and asecond surface on the opposed side of the display layer from the firstsurface, the display layer being provided with means for applying anelectric field between the first and second surfaces, the display layerfurther comprising an electrophoretic fluid comprising a fluid andfirst, second, third and fourth types of particles dispersed in thefluid, the first, second, third and fourth types of particles havingrespectively first, second, third and fourth optical characteristicsdiffering from one another, the first and third types of particles bearcharges of one polarity and the second and fourth types of particlesbear charges of the opposite polarity, the method comprising: a)applying to all pixels a first electric field having a high magnitude,thereby driving the second type of particles towards the viewing surfaceand causing the display layer to display the second opticalcharacteristic at the viewing surface; b) thereafter applying to allpixels a second electric field having a lower magnitude than, and apolarity opposite to, the first electric field, thereby driving thethird type of particles towards the viewing surface and causing thedisplay layer to display the third optical characteristic at the viewingsurface; and c) thereafter applying to at least one pixel of the displayan electric field to cause the pixel to display an opticalcharacteristic different from the optical characteristic displayed atthe end of step b).
 8. The method of claim 7 wherein step c) is effectedby at least one of the following: (i) applying to at least one pixel ofthe display a high electric field having the same polarity as the firstelectric field, thereby causing the pixel to display the second opticalcharacteristic; (ii) applying to at least one pixel of the display ahigh electric field having the opposite polarity from the first electricfield, thereby causing the pixel to display the first opticalcharacteristic; and (iii) applying to at least one pixel of the displaya high electric field having from the opposite polarity from the firstelectric field, followed by applying to the pixel a low electric fieldof the same polarity as the first electric field, thereby causing thepixel to display the fourth optical characteristic.
 9. The method ofclaim 7, further comprising applying a shaking waveform prior to step(a).